The 915 was in production from 1972 through 1986, so there were several versions, with 3 different ring & pinion ratios available. If you knew the year of car it came out of it would help! Even if you don't, there is a 'series' number stamped on the bottom rib, and it will appear as 915/xx, and there is also a serial number, too, close by. As far as the hp and torque ratings go, I'll have to look that up in Paul Frere's book. But, with a cooler, pump, and spray squirters, the competition models were good for some big numbers! Get a copy of Bruce Anderson's 'Porsche 911 Performance Handbook' ... he has a chapter with plenty of pictures and ideas! You should visit PowerHaus II at URL: www.phii.net and take a look at their gear offerings!

Thanks - I don't have ready access to the gearbox (I'm in a different state at the moment) so I can't check, so I'll have to either wait until I can get to it, or simply 'count the teeth'.
FWIW, I'll be needing a diff ratio of about 4.3:1, and it's going into a car that should have about 500hp/380ft-lbs, and weigh about 1500lbs, with driver.

I wouldn't use the 915 if you're running an actual 500 HP. You can break them with a 200 HP 3.0L SC if you dump the clutch too many times (I have a friend who's done it twice while dragracing). You might consider a 930 4spd, although the gearing may need to be changed. The HP/torque specs you give hint at a high strung (turbo?) engine. If this is the case then takeoff shouldn't be the problem, but a 1-2 hard shift with all the power down might be. I'm an advicate of overkill. You know your driving habbits.

V8 torque is a little different from the 6 cyl.
The V8 has 4 "pulses" per revolution instead of the 3 "pulses" that the 6 cyl has.
So from a technical stand point there is less time between crank pulses and, as a result, it is smoother. This "smoother" will let you increase the torque at the crank with less of a shock factor on the rest of your components. This could / might explain why these transmissions break with 6cyl's with less power.
With that said I have been told/read the following

1972-76 the 915 was good for 450ft/lbs (v8)For 1977-on the main shaft was strenghtend and it could now survive 600 ft/lbs (v8)
Your weak link will be your clutch set up. If you abuse it, it will go away quicky and smell bad.
Anyway, the 911sc is good for about 325 ft/lbs but the PP and the disc can be up graded to handle about 450 ft/lbs
I gave you the torque figures because that what seems to be important here
You also might want to get a taller R&P because you will be stirring the gears very quickly and this box is not renowned for it's lightning fast shifts from 1st-2nd.
Hope it helps
s

H2O911 - Thanks, but I need the 4.3:12 diff ratio to make the engine rev out to near max revs in 5th gear - ~290kph.
Since it's a pure racing car, I'll only be doing the 1 - 2 shift once, then using the rest of the gears as needed. In fact, I doubt if I'll be using 5th gear much, as there's not that many tracks here in Aus that have a long enough straight to get to 290kph!
FWIW, I'm traditionally pretty easy on the gearbox, eg, I've had a 10,000rpm 180hp Suzuki GTi 1300cc twin cam running through a Toyota K-50 Corolla early model gearbox with no problems. (New engine in now though, only has ~140hp due to new rules)

Thanks - I'll have a look for the ratios.
The engine is a Toyota Lexus 1UZFE all alloy 4 litre quad cam V-8. With not a lot of mods, I'm expecting a good 500hp from it.
Bloody strong - they have a six bolt bottom end - stock!